A fuel cell stack has a structure in which several tens to several hundreds of unit cells are stacked. Each unit cell includes a polymer electrolyte membrane which moves a hydrogen cation (proton). An air electrode (cathode) and a fuel electrode (anode) are applied to both surfaces of the electrolyte membrane as catalyst layers such that hydrogen may react with oxygen. A gas diffusion layer is stacked outside the air electrode and the fuel electrode. A bipolar plate is stacked outside the gas diffusion layer to supply fuel and discharge water through a flow channel.
During an initial operation of the fuel cell stack after being assembled, activity of the fuel cell stack is reduced in an electrochemical reaction. Accordingly, it is necessary to perform a stack activation process in order to maximize the initial performance.
This stack activation process is also called “pre-conditioning” or “break-in”, which activates a catalyst that does not react and secures a hydrogen ion channel by sufficiently hydrating electrolytes contained in the electrolyte membrane and electrodes.
In order for the fuel cell stack to exhibit a normal performance after being assembled, the stack activation process is performed for securing a three-phase electrode reaction area, removing impurities from the polymer electrolyte membrane or electrodes, and improving ionic conductivity of the polymer electrolyte membrane.
For example, in a conventional method for stack activation, a process of discharging a high-current density (1.2 or 1.4 A/cm2) for a prescribed amount of time (minutes) and a process in which pulse discharge is performed in a shutdown state for a prescribed amount of time are repeated several tens of times. However, the activation process through the pulse discharge has a problem in that the amount of hydrogen used therein as well as the processing time increases.
In order to resolve this problem, a method for activating a fuel cell stack using vacuum wetting has been proposed. In the method for activating a fuel cell stack using vacuum wetting, a process of discharging a high-current density and a vacuum wetting process in which a vacuum is generated in the fuel cell stack in a shutdown state are alternately repeated several times to several tens of times.
The above method may reduce the time required for activation and the amount of hydrogen used, compared to an activation method performed using only an existing constant current or electric potential, but due to limitations of activation equipment for stack activation (including an electronic load), the amount of time during which the activation equipment has to be used is relatively long.
Consequently, when production of fuel cell stacks increases in the future, the stack activation may delay a production time of the fuel cell stacks due to the limitations of the activation equipment. Accordingly, there exists a need for an activation process that can accelerate the activation time of fuel cell stack, and simultaneously, reduce the amount of hydrogen used for activation in order to prepare for the mass production of fuel cell vehicles.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.